Railgun

How a railgun works

A railgun is, electrically, the simplest thing imaginable: a single loop of wire with one side that can slide. Two parallel conducting rails are bridged at the breech by a movable armature — a metal slug (or a conductive sabot behind the actual projectile). Close the circuit and an enormous pulsed current — typically 1 to 6 million amps — flows up one rail, across the armature, and back down the other rail.

That current loop generates a magnetic field in the gap between the rails. By the right-hand rule the field points perpendicular to the plane of the loop — straight up or down between the two rails. The armature carries current across that field. A current in a magnetic field feels a force, and here the force points along the rails, away from the breech. So the field made by the current pushes on the current that made it, and the armature accelerates down the barrel, dragging the projectile with it.

The key contrast with a conventional gun: there is no chemical propellant and no expanding gas. The projectile is shoved entirely by the magnetic Lorentz force on the moving current bridge. Cut the current at the muzzle and the projectile flies free at hypersonic speed.

The governing physics

The microscopic force on the armature is the magnetic part of the Lorentz force. For a straight conductor of length L carrying current I in a field B:

F = I L × B        (magnitude F = B·I·L when L ⊥ B)

That is honest but inconvenient — B between two finite rails is non-uniform and ugly to integrate. Engineers use an equivalent, far cleaner form derived from energy. The magnetic energy stored in the rail loop is

U = ½ L·I²

where L is the loop's inductance. As the armature slides outward by dx, it lengthens the loop, so the inductance grows. The force is the gradient of stored energy with respect to position at constant current:

F = +½ I² · (dL/dx) = ½ L'·I²

Here L' = dL/dx is the inductance gradient ("L-prime"), the inductance added per metre of rail travel. For typical rail geometries L' ≈ 0.4–0.6 µH/m and is roughly constant, so it can be measured once and reused. Two facts fall straight out of this equation:

• Force scales with the square of current. Double I and the thrust quadruples. This is why railguns chase megaamp currents — and why current, not voltage, is the design driver.
• Force is independent of how far the armature has travelled (as long as L' is constant). The push is the same at the breech as near the muzzle, so the projectile accelerates uniformly down the bore.

Constant force over a barrel of length ℓ gives a muzzle velocity from the work–energy theorem, F·ℓ = ½ m v²:

v = √( 2·F·ℓ / m ) = √( L'·I²·ℓ / m )

Worked example — sizing a shot

Take a frame in the ballpark of the US Navy prototype: I = 3 MA peak, L' = 0.5 µH/m, projectile mass m = 3 kg, barrel length ℓ = 10 m.

F = ½ · L' · I²
  = ½ · (0.5×10⁻⁶ H/m) · (3×10⁶ A)²
  = ½ · 0.5×10⁻⁶ · 9×10¹²
  = 2.25×10⁶ N   (≈ 230 tonnes of force)

a = F / m = 2.25×10⁶ / 3 ≈ 7.5×10⁵ m/s²   (≈ 76,000 g)

v = √(2·a·ℓ) = √(2 · 7.5×10⁵ · 10) ≈ 3,870 m/s

E = ½ m v² = ½ · 3 · 3870² ≈ 2.2×10⁷ J  (22 MJ)

That is roughly Mach 11 and tens of megajoules of muzzle energy from a single shot — and it explains why the projectile must survive ~10⁴–10⁵ g of acceleration, a brutal constraint on any guidance electronics packed inside it. Real guns run lower because the current is a decaying pulse, not a flat 3 MA, and because friction and ablation bleed energy; demonstrated speeds land around 2.5 km/s.

Key regimes and conditions

A railgun only works inside a narrow operating window, bounded by what the materials can tolerate:

• Solid-armature regime (below ~1.5–2 km/s). The armature stays in metal-to-metal sliding contact. Clean, efficient current transfer, lowest erosion.
• Transition / velocity skin effect (~2 km/s). At high speed the current crowds into the rear trailing edge of the armature contact instead of spreading out — the velocity skin effect. The contact patch overheats and starts to melt.
• Plasma-armature regime (above ~3 km/s). The molten contact vaporises and the current jumps the gap as a plasma arc. A plasma armature can push the projectile to higher speeds but its arc roots blowtorch the rail faces, causing severe ablation and limiting barrel life.
• Rail-repulsion limit. The two current-carrying rails feel their own mutual force — the same physics as two parallel wires — and being antiparallel, they repel. The rails must be clamped hard enough to resist megaamp-driven spreading, or the bore opens and the armature loses contact.

Railgun vs conventional gun vs coilgun

Property	Railgun	Conventional (chemical) gun	Coilgun (Gauss gun)
Driving force	Lorentz force on armature current, F = ½ L' I²	Pressure of expanding propellant gas	Switched solenoid attraction (linear motor)
Energy stored as	Electrical (capacitors / pulsed generator)	Chemical (gunpowder, smokeless powder)	Electrical (capacitors)
Electrical contact with projectile	Yes — current flows through the armature	None	None (contactless)
Typical muzzle velocity	2,000–3,000 m/s	900–1,800 m/s	~100–1,000 m/s (lab scale)
Velocity ceiling cause	Rail ablation, plasma transition, skin effect	Gas can't expand faster than its sound speed (~2 km/s)	Coil timing & magnetic saturation
Main wear mechanism	Rail erosion, contact melting	Barrel throat erosion from hot gas	Almost none (no contact)
Peak input demand	Megaamp current pulse, < few ms	—	Multi-kA per coil, staged
Logistics	No explosive magazine; needs heavy power plant	Volatile propellant stockpile aboard	No explosives; modest power

Real-world figures and costs

The flagship program was the US Navy Electromagnetic Railgun, developed by BAE Systems and General Atomics from the mid-2000s. Documented figures:

• Muzzle energy: demonstrated up to 33 MJ in test firings, with a 64 MJ design goal (1 MJ ≈ the kinetic energy of a 1-tonne car at 160 km/h — so 32 of them).
• Muzzle velocity: ~2,500 m/s (Mach 7+) with a ~3.2 kg projectile.
• Range goal: ~180 km (≈100 nautical miles), far beyond a 5-inch naval gun's ~30 km.
• Pulse power: firing one shot dumps tens of megajoules in a few milliseconds — instantaneous power in the gigawatt range, sourced from charged capacitor banks or flywheel/homopolar generators.
• Barrel life: the limiting cost. Early bores survived only tens of full-energy shots before rail erosion forced replacement; reaching the desired rate of fire (~10 rounds/min) sustainably was never demonstrated, and the Navy paused funding in 2021 after spending roughly half a billion dollars.

The projectile itself is cheap compared to a guided missile (no propellant, no rocket motor), which was the original appeal. The expensive parts are the power plant, the thermal management, and the consumable rails.

Where it shows up

• Naval and ground artillery. The headline application — long-range, high-velocity kinetic strike with a non-explosive magazine that's safer to store aboard ship.
• Hypervelocity impact research. Smaller lab railguns launch grams to a few km/s to simulate micrometeoroid and orbital-debris impacts on spacecraft shielding.
• Inertial-fusion and high-pressure physics. Railguns and the related rail-driven flyer plates create the pressures needed for equation-of-state experiments.
• Proposed space launch. Studied (e.g. by NASA) as a first-stage mass driver to fling payloads up a long inclined rail; attractive in principle but the g-loads rule out fragile or crewed cargo, and atmospheric heating at launch speed is severe.
• Aircraft-carrier catapults (cousin technology). The Navy's EMALS launcher uses a linear induction motor — a coilgun-family device, not a railgun — to throw aircraft, the same electromagnetic-launch lineage.

Common misconceptions and edge cases

• "The projectile must be magnetic." No. The armature only needs to conduct — the force is on the current, not on a ferromagnet. (A coilgun is the one that wants a ferromagnetic slug.)
• "Higher voltage makes it more powerful." Force depends on current squared, not voltage. Voltage matters only insofar as it pushes current up fast through the loop's inductance; the work is done by amps.
• "The rails attract each other and clamp the armature." The rails carry current in opposite directions, so they repel — they try to fly apart, which is a structural headache, not a help.
• "It's frictionless because it's electromagnetic." The armature is in hard sliding contact under megaamp current; friction and ohmic heating melt metal. Contactless launch is the coilgun, not the railgun.
• "More barrel length always means more speed." Only while the current pulse lasts and the contact survives. Past a few metres the pulse has decayed and ablation dominates, so extra rail adds erosion, not velocity.
• "Recoil is eliminated." Momentum is still conserved. The reaction force appears at the breech where the current enters, so the mount feels a recoil equal and opposite to the projectile's momentum, just as with a chemical gun.